Go to Clients/Agents -- Agents -- Agents and click the Pencil icon next to the agent you want to edit.
Make any necessary changes to their profile and hit Save Agent at the bottom.
The process for Clients is much the same, just go to Clients/Agents - Clients - Clients instead.
From the order view screen.
You can edit a client or an agent from the order view screen by clicking the Pencil button next to their contact info for the order.